1. Field of the Invention
The present invention relates to a wireless communication system. More particularly, the present invention relates to an apparatus and method for HandOver (HO)-related Radio Link Failure (RLF) detection in a wireless communication system.
2. Description of the Related Art
Research is being conducted to develop a next generation communication system, also referred to as a 4th Generation (4G) communication system or an International Mobile Telecommunications-Advanced (IMT-Advanced) communication system. Representative examples of such a next generation communication system include a communication system based on an Institute of Electrical and Electronics Engineers (IEEE) 802.16m standard (referred to as an IEEE 802.16m system) and a communication system based on a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Advanced standard (referred to as an LTE Advanced system), both of which are under development.
One of the requirements of the next generation communication system is a Self-Organizing Network (SON). A SON allows for automated adjustments to a wireless communication system to optimize performance while providing more efficient Operation and Maintenance (O&M). One of the key functionalities of SON is Mobility Robustness Optimization (MRO). Radio Link Failures (RLFs) may be caused by incorrect or non-optimized HandOver (HO) parameter settings, a Physical Cell Identifier (PCI) collision, etc. RLFs caused by incorrect or non-optimized HO parameter settings have a combined impact on user experience and network resources. Therefore, an objective of MRO should be to reduce the number of HO-related RLFs. Consequently, the cause of the HO-related RLFs needs to be determined so that corrective action may be taken.
HO-related RLFs are described in the 3GPP TR 36.902 v9.0.0 specification, the entire disclosure of which is hereby incorporated by reference. HO-related related RLFs may be categorized as one of failures due to HO to an incorrect cell, failures due to too late HO triggering, and failures due to too early HO triggering. These HO-related RLFs typically occur due to HO parameter settings that are incorrect or not optimized. The 3GPP TR 36.902 v9.0.0 specification further suggests schemes to detect the HO-related RLFs. In addition, schemes to detect the HO-related RLFs have been suggested in 3GPP proposal R3-091357, the entire disclosure of which is hereby incorporated by reference.
Examples of the HO-related RLF categories will be described below with reference to FIGS. 1-4, as will the schemes for detecting the HO-related RLFs proposed by TR 36.902 v9.0.0 and R3-091357.
FIG. 1 illustrates an example of a scenario of HO to an incorrect cell according to the conventional art.
Referring to FIG. 1, a User Equipment (UE) 100, an evolved Node B (eNB) A 110 servicing Cell A 111, an eNB B 112 servicing Cell B 113, and an eNB C 114 servicing Cell C 115 are shown. The service coverage areas of Cell A 111, Cell B 113, and Cell C 115 overlap. Here, the UE 100 has an established connection with the eNB A 110 and is moving into the service coverage area of Cell C 115. Due to the HO parameter settings not being optimized, HO is performed to eNB B 112 instead of eNB C 114. However, the UE 100 experiences an RLF shortly after the UE 100 successfully performs HO to the eNB B 112. Since the UE 100 is within the service coverage area of Cell C 115 and outside or at the outer limits of coverage area of Cell B 113, the UE 100 reestablishes the connection with eNB C 114 instead of eNB B 112.
TR 36.902 v9.0.0 does not propose a scheme for detecting an RLF due to HO to an incorrect cell. The scheme for detecting an RLF due to HO to an incorrect cell proposed by TR R3-091357 includes the eNB B 112 receiving an RLF report from eNB C 114 since the eNB C 114 considers the RLF to be caused by a too late HO. Here, eNB B 112 keeps the context concerning this UE 100 after it has completed the HO from eNB A 110. In this case, eNB B 112 can determine that the RLF is caused by an HO to a wrong cell and then send an RLF report to eNB A 110.
FIG. 2 illustrates an example of a too late HO triggering scenario according to the conventional art.
Referring to FIG. 2, a UE 100, an eNB A servicing Cell A 111, and an eNB B 112 servicing Cell B 113 are shown. The service coverage areas of Cell A 111 and Cell B 113 overlap. Here, the UE 100 has an established connection with the eNB A 110 and is moving into the service coverage area of Cell B 113. However, the UE 100 leaves the service coverage area of Cell A 111 before the HO to eNB B 112 is initiated or before the HO is complete, and thus experiences an RLF with eNB A 110. Since the UE 100 is no longer within the service coverage area of Cell A 111 and is instead within the service coverage area of Cell B 113, the UE 100 reestablishes the connection with eNB B 112 instead of eNB A 110.
The scheme for detecting a too late HO-related RLF proposed by TR 36.902 v9.0.0 includes the eNB B 112 sending an RLF report to eNB A 110 after the UE 100 reestablishes the connection with eNB B 112 instead of eNB A 110. In contrast, the scheme for detecting a too late HO-related RLF proposed by R3-091357 includes the UE 100 sending an RLF report to the eNB B 112 after the UE 100 reestablishes the connection with eNB B 112, and then the eNB B 112 sends an RLF report to eNB A 110.
FIG. 3 illustrates an example of a too early HO triggering scenario according to the conventional art.
Referring to FIG. 3, a UE 100, an eNB A 110 servicing Cell A 111, and an eNB B 112 servicing Cell B 113 are shown. The service coverage areas of Cell A 111 and Cell B 113 overlap. Here, the UE 100 has an established connection with the eNB A 110 and is moving into the service coverage area of Cell B 113. An HO is then successfully performed to the eNB B 112. However, the UE 100 experiences an RLF shortly after the HO. Since the UE 100 is within the service coverage area of Cell A 111 and outside or at the outer limits of coverage area of Cell B 113, the UE 100 reestablishes the connection with eNB A 110 instead of eNB B 112.
The scheme for detecting a too early HO-related RLF proposed by TR 36.902 v9.0.0 includes the eNB B 112 ignoring an RLF report received from eNB A 110, if eNB B 112 sent a UE 100 Context Release message to eNB A 110 that is related to the completion of an incoming HO for the same UE 100 within the last Tstore—UE—cntxt seconds.
In contrast, the scheme for detecting a too early HO-related RLF proposed by R3-091357 includes the eNB A 110 considering the reestablished connection with eNB A 110 instead of eNB B 112 as a too late HO-related RLF and sending an RLF report to eNB B 112. At this time, the eNB B 112 keeps a context related to the UE 100 that the eNB B 112 sent in a UE 100 Context Release message to eNB A 110 during the previous HO procedure. Thus, eNB B 112 can determine that the RLF report sent by eNB A 110 is caused by a too early HO triggering and not a too late HO triggering. Then, eNB B 112 sends an RLF report back to eNB A 110. Here, eNB B 112 will not optimize its HO parameter settings while eNB A 110 will optimize its HO parameter settings.
A flow diagram for detecting a too early HO-related RLF according to R3-091357 is described below with reference to FIG. 4.
FIG. 4 illustrates a flow diagram for detecting a too early HO-related RLF according to the conventional art.
Referring to FIG. 4, an HO procedure is performed in step 400. The HO procedure of step 400 may correspond to a procedure that occurs in FIG. 10.1.2.1.1-1 of the 3GPP TS 36.300 v9.1.0 specification, the entire disclosure of which is hereby incorporated by reference. Alternatively, the HO procedure of step 400 may correspond to a procedure that occurs in FIG. 5.5.1.2.2-1 of the 3GPP TS 23.401 v9.2.0 specification, the entire disclosure of which is hereby incorporated by reference.
Once the HO procedure of step 400 is finished, a UE 100 is serviced by the eNB B 112 in step 402. However, the UE 100 experiences an RLF shortly after the HO in step 404. Since the UE 100 is within the service coverage area of Cell A 111 and outside or at the outer limits of coverage area of Cell B 113, the UE 100 reselects the eNB A 110 as a serving eNB instead of eNB B 112 in step 406. The UE 100 sends a Radio Resource Control (RRC) Connection Reestablishment Request (RRCConnectionReestablishmentRequest) message to the eNB A 110 in step 408 in order to reestablish the connection with eNB A 110.
The eNB A 110 sends an RRC Connection Reestablishment (RRCConnectionReestablishment) message to the UE 100 in step 410. In response, the eNB A 110 sends an RRC Connection Reestablishment Complete (RRCConnectionReestablishment Complete) message to the eNB A 110 in step 412.
The eNB A 110 considers the reestablished connection of the UE 100 with eNB A 110 instead of eNB B 112 as a too late HO-related RLF and sends an RLF report to eNB B 112 in step 414. The eNB B 112 has maintained the context related to the UE 100 that was sent in a UE 100 Context Release message to eNB A 110 during the HO procedure of step 400. Thus, eNB B 112 determines that the RLF report sent by eNB A 110 is caused by too early HO triggering and not by too late HO triggering. The eNB B 112 then sends an RLF report back to eNB A 110 in step 416 indicating that the RLF of step 404 was caused by too early HO triggering. Here, eNB B 112 will not optimize its HO parameter settings while eNB A 110 will optimize its HO parameter settings.
In the HO-related RLF categories described above, neither TR 36.902 v9.0.0 nor R3-091357, provide an adequate solution for detecting the various HO-related RLFs. In particular, neither TR 36.902 v9.0.0 nor R3-091357 provides an adequate solution for detecting too early HO-related RLF. The scheme for detecting too early HO-related RLF in TR 36.902 v9.0.0 is incomplete. In the scheme for detecting too early HO-related RLF proposed in R3-091357, it is a target eNB (eNB B 112 in FIG. 3) that analyzes the cause of the RLF. Having the target eNB analyze the cause of the RLF requires that additional signaling be sent from target eNB to the source eNB (eNB A 110 in FIG. 3).
Therefore, a need exists for an apparatus and method for detecting HO-related RLF in a wireless communication system.